1. Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus that includes the image sensor.
2. Description of the Related Art
An image capturing apparatus that performs focus detection according to a pupil division phase difference method using a two-dimensional image sensor in which a microlens is formed in each pixel has been proposed as one method for detecting a focus state of a taking lens.
For example, U.S. Pat. No. 4,410,804 discloses an image capturing apparatus that uses a two-dimensional image sensor in which each pixel has one microlens and a plurality of divided photoelectric conversion portions. The divided photoelectric conversion portions are configured to receive light that is transmitted through different regions of an exit pupil of the taking lens through a single microlens, to thus perform pupil division. An image shift amount is determined based on the respective signals for light received by the divided photoelectric conversion portions, focus detection is performed, and signals for light received by the divided photoelectric conversion portions are added together to acquire an imaging signal.
The disclosure of U.S. Pat. No. 4,410,804 is not limited to focus detection, and also discloses that a 3D image can be acquired by displaying, in a separate manner for the right eye and the left eye, parallax signals with respect to light received with photoelectric conversion portions that are divided into left and right in each pixel.
Further, Japanese Patent Laid-Open No. 2009-015164 discloses an apparatus that includes a plurality of pixel columns in which dividing positions of photoelectric conversion portions are different, and that reduces an imbalance in pixel output by selecting an optimal pixel column in accordance with an eccentricity error of a microlens or a pupil change that depends on the kind of interchangeable lens or the image height.
However, the light-receiving sensitivity decreases at a separation band that is formed to separate a plurality of photoelectric conversion portions. There is thus the problem that the separation band forms a low sensitivity band in accordance with the exit pupil shape on the exit pupil of the taking lens through a microlens, and unnatural blurring occurs in a captured image. Further, according to the technology disclosed in Japanese Patent Laid-Open No. 2009-015164, there is the problem that when the f-number of the taking lens is narrowed, the image quality of a captured image deteriorates as a result of a decrease in the pixel output. Furthermore, since all adjacently arranged pixels are divided in the same manner, the pixel output decreases in a continuous manner for a plurality of pixels, and consequently the image quality deteriorates further. Although a decrease in the pixel output can be prevented by setting the f-number of the taking lens to a bright setting when performing focus detection, since it is not possible to apply a restriction to the f-number that is a principal factor for exposure control when capturing an image, this is a significant problem.